The oil industry utilizes a variety of methods for recovering oil from subterranean formations. Initially, oil is produced from a formation by pressure depletion. In this method, the differential pressure between the formation and a production well or wells forces the oil contained within the formation toward a production well where it can be recovered. Typically, only about 10 to about 35 percent of the oil which is contained within a formation can be recovered from a formation using pressure depletion. This leaves a large quantity of oil within the formation. Additionally, some formations contain oil which is too viscous to be efficiently recovered from the formation using pressure depletion methods. Because of the need to recover a larger percentage of the original oil-in-place from a formation, several methods have been developed which facilitate the recovery of oil which could not be recovered using pressure depletion techniques. These methods are sometimes hereinafter referred to as "recovery techniques."
Water flooding is the most frequently utilized enhanced recovery technique. In water flooding, water is injected into a formation through an injection well. The injected water moves within the formation and mobilizes the accumulations of oil contained therein. The mobilized oil is moved within the formation toward a production well or wells where it is recovered. Water flooding may be used alone, or it may be combined with other techniques which are performed concurrently with the water flood or subsequent to it. The intent of the techniques are to improve the displacement efficiency of the method so that more of the original oil-in-place can be efficiently recovered from the formation. For example, water flooding may be combined with the injection of a gaseous phase, such as nitrogen, carbon dioxide, air, oxygen, or flue gas. In general, the injected gases mobilize the oil remaining within the formation and make it easier to move the remaining oil toward a production well where it can be recovered. Additionally, materials which tend to block any high permeability paths within a formation may be mixed with the injected water, or chemicals which lower the interfacial tension of oil and water may be added to the injected water to improve the recovery of oil from the formation.
Water flooding is effective at recovering additional hydrocarbons from a formation and is relatively easy to initiate on a field which is operating under pressure depletion. Additionally, the capital requirements for water flooding are less when compared to other enhanced recovery techniques. The displacement efficiency of water flooding, however, is typically not as high as the displacement efficiency of other enhanced recovery techniques, such as in-situ combustion.
Several enhanced recovery techniques utilize thermal energy to enhance the recovery of hydrocarbons from a formation. One such technique utilizes steam injection to enhance the recovery of oil from a formation. Steam injection is generally used to enhance the recovery of oil from formations containing heavy oil. The steam is injected into the formation through an injection well. Once within the formation, the steam heats the oil and reduces the oil's viscosity. This helps the oil to flow to a production well where it can be recovered from the formation. Additionally, the steam increases the pressure within the formation and assists in pushing the oil toward a production well.
Steam injection recovery techniques require on-site facilities for producing the steam utilized by the technique. Also, heavy oil deposits are often underlain by a water layer. The steam injected into such a formation will preferentially sweep the water layer which underlays the formation. This will cause large amounts of steam to be utilized to recover heavy oil from such a formation. Also, a large percentage of the heavy oil contained within the formation is not recoverable due to the failure of the steam to sweep a large region of the formation. Further, for formations located below approximately 600 meters, the heat losses as the steam travels down the wellbore are so large that it becomes economically impractical to use steam injection to enhance the recovery of oil from such a formation.
In-situ combustion is another enhanced recovery technique which, at least in part, utilizes thermal energy to enhance the recovery of oil from a formation. Typically, with in-situ combustion, a gaseous oxidant such as air, oxygen-enriched air, or high purity oxygen is introduced into the formation through an injection well. The injected gaseous oxidant mixes with a portion of the oil present within the formation to form a combustible mixture which may spontaneously ignite if the formation conditions are appropriate. If required, the combustible mixture can be artificially ignited using devices known to one of ordinary skill in the art. The combustion of the gaseous oxidant with oil present within the formation is an exothermic reaction which liberates heat and produces flue gas and steam. At least a portion of the heat liberated is transferred to the remaining uncombusted oil contained within the formation. The heat transferred to the oil mobilizes it and facilitates its movement toward a production well. Additionally, as combustion occurs within the formation, the gases produced tend to increase the pressure in a region of the formation near the combustion front. The differential pressure which results within the formation will assist in moving remaining oil toward a region of lower pressure, such as a production well. Further, the combustion products which result from the in-situ combustion, such as flue gas, are known to be beneficial to the recovery of oil from a formation.
In-situ combustion, which utilizes an injected gaseous oxidant, can be utilized on various types of formations, including formations containing heavy oil, medium oil, or light oil. In-situ combustion provides an efficient means for improving the displacement efficiency of oil from a formation. However, the equipment utilized to inject the gaseous oxidant is typically expensive to install and maintain. Also, there is a tendency for the injected gaseous oxidant to sweep only the upper regions of vertically thick formations. Further, the technique is complicated and requires a detailed and complete understanding of the properties and characteristics of a reservoir to carry it out. The above factors make it impracticable to utilize an injected gaseous oxidant to create in-situ combustion within many formations.
U.S. Pat. No. 4,867,238 to Bayless et al., discloses another enhanced recovery technique which at least in part utilizes thermal energy to aid in recovering oil from a formation. The patent discloses the injection of a hydrogen peroxide solution into a formation containing viscous oil. The hydrogen peroxide decomposes by an exothermic reaction to produce oxygen and water. The oxygen produced is available to further react with oil contained in the formation to produce carbon dioxide, carbon monoxide, water, and heat. Hydrogen peroxide is expensive and requires special containers for storage and transport. Additionally, hydrogen peroxide is a highly reactive chemical which can corrode oil field tubulars. Further, it also may be difficult to control the decomposition of hydrogen peroxide within a typical formation.
What is desired is a method for recovering oil from a formation which provides a displacement efficiency similar to that characteristic of in-situ combustion techniques, together with the simplicity of operation characteristic of water flooding. The method should be capable of being utilized on formations which contain light, medium, or heavy oil.
As used herein, the following terms shall have the following meanings:
(a) "sweep" refers to the region of a formation contacted by the fluid introduced into the formation. The sweep of the formation is measured as a percentage of the formation contacted. With in-situ combustion, the total sweep results from introduced fluid and in-situ combustion products sweeping the formation. The total sweep is the product of the sweep in the areal and vertical directions; PA1 (b) "flue gas" refers to the gaseous mixture which results from the combustion of a hydrocarbon with air. The exact chemical composition of flue gas depends on many variables, including but not limited to, the combusted hydrocarbon, the combustion process oxygen-to-fuel ratio, and the combustion temperature; PA1 (c) "combustion" refers to a rapid exothermic reaction which occurs when hydrocarbons are reacted with an oxidant. When oxygen is the oxidant, the reactions typically produce carbon dioxide (CO.sub.2), carbon monoxide (CO), and water (H.sub.2 O). The reactions may also produce nitrogen oxides, sulfur oxides, and other reaction products; PA1 (d) "in-situ combustion" refers to combustion which occurs within a subterranean formation between oil contained within the formation and an oxidant; PA1 (e) "heavy oil" is crude oil having an API gravity of less than 20.degree. C.; PA1 (f) "medium oil" is crude oil having an API gravity of between 20.degree. and 30.degree. C.; PA1 (g) "light oil" is crude oil having an API gravity of greater than 30.degree. C.; PA1 (h) "decomposition temperature" is the temperature at which a substantial weight percentage of a salt introduced into the formation will begin to decompose and to generate an oxidant; PA1 (i) "ignition temperature" is the temperature at which oil contained within a subterranean formation would be capable of igniting if provided with a sufficient quantity of a suitable oxidant; and PA1 (j) a "pore volume" refers to the open space in a rock formation not occupied by solid mineral matter.